133 research outputs found
Two new bright Ae stars
Two newly identified Ae stars, nu Cyg and kappa UMa, were discovered in the
course of the Magnetic Survey of Bright MS stars (Monin et al. 2002). We pre
sent their Halpha profiles along with measurements of their equivalent width
and parameters of emission features. Emission in the Halpha line of nu Cyg is
variable on a time scale of 3 years. kappa UMa exhibits weak emission which is
rather stable. The emission is thought to arise from a circumstellar disk, and
we have estimated the size of that disk.Both new emission stars are IRAS
sources. Their IR color excesses are consistent with those of classical Ae
stars. Thus, nu Cyg and kappa UMa appear not to belong to the class of Herbig
Ae/Be stars. We argue that the frequency of Ae stars may be underestimated due
to the difficulty of detection of weak emission in some A stars.Comment: 6 pages,3 figures, submitted to A&
A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-mass stars
that exhibit an 8 micron excess in either the Galactic Legacy Infrared
Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high
resolution optical spectra to identify stars surrounded by warm circumstellar
dust. From these data we determine projected stellar rotational velocities and
estimate stellar effective temperatures for the sample. We estimate stellar
ages from these temperatures, parallactic distances, and evolutionary models.
Using MIPS [24] measurements and stellar parameters we determine the nature of
the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha
emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses
arise in circumstellar disks, we use the Halpha fluxes to model and estimate
the relative contributions of dust and free-free emission. Six stars exhibit
Halpha fluxes that imply free-free emission can plausibly explain the infrared
excess at [24]. These stars are candidate classical Be stars. Nine stars
exhibit Halpha emission, but their Halpha fluxes are insufficient to explain
the infrared excesses at [24], suggesting the presence of a circumstellar dust
component. After the removal of the free-free component in these sources, we
determine probable disk dust temperatures of Tdisk~300-800 K and fractional
infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be
pre-main-sequence stars with transitional disks undergoing disk clearing. Three
of the four sources showing Halpha absorption exhibit circumstellar disk
temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm
debris disk candidates. One of the four Halpha absorption sources has K-[24]>
3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap
The Circumstellar Disk of HD 141569 Imaged with NICMOS
Coronagraphic imaging with the Near Infrared Camera and Multi Object
Spectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4'')
radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected
light image at 1.1 micron shows the disk oriented at a position angle of 356
+/- 5 deg and inclined to our line of sight by 51 +/- 3 deg; the intrinsic
scattering function of the dust in the disk makes the side inclined toward us,
the eastern side, brighter. The disk flux density peaks 185 AU (1.''85) from
the star and falls off to both larger and smaller radii. A region of depleted
material, or a gap, in the disk is centered 250 AU from the star. The dynamical
effect of one or more planets may be necessary to explain this morphology.Comment: 4 pages, LaTeX with emulateapj.sty and epsfig.sty, 4 postscript
figures, Accepted to ApJ Letter
NGC 2401: A template of the Norma-Cygnus Arm's young population in the Third Galactic Quadrant
Based on a deep optical CCD (UBV(RI)_C) photometric survey and on the
Two-Micron All-Sky-Survey (2MASS) data we derived the main parameters of the
open cluster NGC 2401. We found this cluster is placed at 6.3 0.5 kpc
(V_O - M_V = 14.0 \pm 0.2) from the Sun and is 25 Myr old, what allows us to
identify NGC 2401 as a member of the young population belonging to the
innermost side of the extension of the Norma-Cygnus spiral--arm in the Third
Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies
in the cluster area revealed it is a B0Ve star; however, we could not confirm
it is a cluster member. We also constructed the cluster luminosity function
(LF) down to and the cluster initial mass function (IMF) for all
stars with masses above M \sim 1-2 M_{\sun}. It was found that the slope of
the cluster IMF is . The presence of a probable PMS star
population associated to the cluster is weakly revealed.Comment: 10 paginas, 11 eps figures, accepted for publication in MNRA
Helium abundance in the most metal-deficient blue compact galaxies: I Zw 18 and SBS 0335-052
We present high-quality spectroscopic observations of the two most-metal
deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine
the helium abundance. The underlying stellar absorption strongly influences the
observed intensities of He I emission lines in the brightest NW component of I
Zw 18, and hence this component should not be used for primordial He abundance
determination. The effect of underlying stellar absorption, though present, is
much smaller in the SE component. Assuming all systematic uncertainties are
negligible, the He mass fraction derived in this component is Y =
0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum)
of SBS 0335-052 allows us to measure the helium mass fraction with a precision
better than 2% -- 5% in nine different regions along the slit. Assuming all
systematic uncertainties are negligible, the weighted mean He mass fraction in
SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678
emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission
line is excluded. The weighted mean helium mass fraction in the two most
metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after
correction for the stellar He production results in a primordial He mass
fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio
of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial
D and 7Li abundances, and supporting the standard big bang nucleosynthesis
theory. For the most consistent set of primordial D, 4He, and 7Li abundances we
derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).Comment: 28 pages, 10 figures. To appear in Ap
The composition of circumstellar gas and dust in 51 Oph
We analyze ISO archive data of the nearby bright emission-line star 51 Oph,
previously classified as a proto-planetary system similar to beta Pic. The
infrared spectrum reveals the presence of gas-phase emission bands of hot
(approx 850 K) CO, CO_2, H_2O and NO. In addition to this, partially
crystalline silicate dust is present. The solid-state bands and the energy
distribution are indicative of dust that has formed recently, rather than of
debris dust. The presence of hot molecular gas and the composition of the
circumstellar dust are highly unusual for a young star and are reminiscent of
what is found around evolved (AGB) stars, although we exclude the possibility
of 51 Oph belonging to this group. We suggest several explanations for the
nature of 51 Oph, including a recent episode of mass loss from a Be star, and
the recent destruction of a planet-sized body around a young star.Comment: 4 pages, 3 figures, accepted by A&A (letters
An Assessment of Dynamical Mass Constraints on Pre-Main Sequence Evolutionary Tracks
[abridged] We have assembled a database of stars having both masses
determined from measured orbital dynamics and sufficient spectral and
photometric information for their placement on a theoretical HR diagram. Our
sample consists of 115 low mass (M < 2.0 Msun) stars, 27 pre-main sequence and
88 main sequence. We use a variety of available pre-main sequence evolutionary
calculations to test the consistency of predicted stellar masses with
dynamically determined masses. Despite substantial improvements in model
physics over the past decade, large systematic discrepancies still exist
between empirical and theoretically derived masses. For main-sequence stars,
all models considered predict masses consistent with dynamical values above 1.2
Msun, some models predict consistent masses at solar or slightly lower masses,
and no models predict consistent masses below 0.5 Msun but rather all models
systematically under-predict such low masses by 5-20%. The failure at low
masses stems from the poor match of most models to the empirical main-sequence
below temperatures of 3800 K where molecules become the dominant source of
opacity and convection is the dominant mode of energy transport. For the
pre-main sequence sample we find similar trends. There is generally good
agreement between predicted and dynamical masses above 1.2 Msun for all models.
Below 1.2 Msun and down to 0.3 Msun (the lowest mass testable) most
evolutionary models systematically under-predict the dynamically determined
masses by 10-30% on average with the Lyon group models (e.g. Baraffe et al.
1998) predicting marginally consistent masses *in the mean* though with large
scatter.Comment: accepted for publication in ApJ (2004
First HARPSpol discoveries of magnetic fields in massive stars
In the framework of the Magnetism in Massive Stars (MiMeS) project, a
HARPSpol Large Program at the 3.6m-ESO telescope has recently started to
collect high-resolution spectropolarimetric data of a large number of Southern
massive OB stars in the field of the Galaxy and in many young clusters and
associations. In this Letter, we report on the first discoveries of magnetic
fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and
confirm the presence of a magnetic field at the surface of HD 105382 that was
previously observed with a low spectral resolution device. The longitudinal
magnetic field measurements are strongly varying for HD 130807 from -100
G to 700 G. Those of HD 122451 and HD 105382 are less variable with
values ranging from -40 to -80 G, and from -300 to -600 G,
respectively. The discovery and confirmation of three new magnetic massive
stars, including at least two He-weak stars, is an important contribution to
one of the MiMeS objectives: the understanding of origin of magnetic fields in
massive stars and their impacts on stellar structure and evolution.Comment: 4 pages, 2 figures, accepted for publication in A&A Lette
The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?
WOH G64 is an unusual red supergiant (RSG) in the Large Magellanic Cloud
(LMC), with a number of properties that set it apart from the rest of the LMC
RSG population, including a thick circumstellar dust torus, an unusually late
spectral type, maser activity, and nebular emission lines. Its reported
physical properties are also extreme, including the largest radius for any star
known and an effective temperature that is much cooler than other RSGs in the
LMC, both of which are at variance with stellar evolutionary theory. We fit
moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar
atmosphere models, determining an effective temperature of 3400 +/- 25 K. We
obtain a similar result from the star's broadband V - K colors. With this
effective temperature, and taking into account the flux contribution from the
aysmmetric circumstellar dust envelope, we calculate log(L/L_sun) = 5.45 +/-
0.05 for WOH G64, quite similar to the luminosity reported by Ohnaka and
collaborators based on their radiative transfer modeling of the star's dust
torus. We determine a radius of R/R_sun = 1540, bringing the size of WOH G64
and its position on the H-R diagram into agreement with the largest known
Galactic RSGs, although it is still extreme for the LMC. In addition, we use
the Ca II triplet absorption feature to determine a radial velocity of 294 +/-
2 km/s for the star; this is the same radial velocity as the rotating gas in
the LMC's disk, which confirms its membership in the LMC and precludes it from
being an unusual Galactic halo giant. Finally, we describe the star's unusual
nebula emission spectrum; the gas is nitrogen-rich and shock-heated, and
displays a radial velocity that is significantly more positive than the star
itself by 50 km/s.Comment: 25 pages, 5 figures; accepted for publication in The Astronomical
Journa
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
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